This invention relates generally to an improved construction of an electric discharge lamp, and more particularly to an electric discharge lamp having auxiliary starting electrode means which includes a more compact thermal switch configuration.
Thermally responsive switch devices are now commonly employed in electric discharge lamps to operate a starting electrode when voltage is initially applied to the lamp. In a typical lamp device, the starting electrode is electrically connected through a resistor to one main lamp electrode physically located at an opposite end of the arc tube so that upon voltage application to the lamp there is caused a high electrical field to be generated between the starting electrode and the other main electrode located adjacent thereto. A glow discharge results thereat followed by formation of the principal lamp discharge between the two main lamp electrodes. The starting electrode serves no further purpose after formation of the principal lamp discharge in a representative metal halide discharge lamp employing such starting means and is commonly short-circuited with respect to the operatively associated adjacent main electrode in order to avoid any significant voltage gradient therebetween which would otherwise deleteriously effect lamp performance such as by removing vaporized constituents from the principal lamp discharge which, in turn, would lead to lamp seal failures. A normally open bimetal switch device is now employed in the above defined lamp operation which is electrically connected to the power source and to the adjacent main lamp electrode. This switch device closes from heat generated by the arc tube while opening when the lamp has been turned off. A more detailed explanation upon both construction and operation of such type discharge lamp is found in U.S. Pat. No. 3,965,387, of Stuart et al. which is assigned to the assignee of the present invention and is herein incorporated by reference. As therein disclosed, such thermal switch means are often used in high intensity discharge lamps wherein they are physically located within the annular space between an inner arc tube formed with fused silica or other refractory glass material and an outer light transmissive envelope also generally formed with a glass material.
The particular bimetal switch device disclosed in the above referenced patent employs a bent strip of the bimetal physically secured at one end to inlead means for one main electrode while having a spring-like conductor secured at the opposite end for engagement with inlead means of the auxiliary electrode. The conductor element thereof is arranged to proceed in a direction along a surface of the bimetal strip and to project forwardly therefrom for engagement with the auxiliary electrode inlead means when thermally actuated. Spring-like flexibility of this conductor element accommodates temperature excursions beyond generally anticipated switch design closure temperatures with the springy conductor temporarily distorting so as to avoid any significant deformation of the bimetal material beyond its elastic limit. Another bimetal switch device now employed in commercial discharge lamps of this type features a bent bimetal strip having right angle bent portions at each end but wherein physical joinder of the bimetal strip to fixed inlead means of the main electrode occurs approximately at a midpoint in the bimetal strip.
Recent discharge lamp developments include a trend toward using smaller sized outer lamp envelopes with existing size arc tubes. Consequently, the annular free space available for lodging lamp components has been significantly reduced making it desirable to have a more compact thermal switch component configuration for utilization therein. Simply reducing the physical size of the above described bimetal switch devices with a shorter bimetal strip is unproductive since the shorter strip will not bend as far and as a result, reliable closure contact may not occur if the bimetal receives insufficient heat from the arc tube under certain lamp operating conditions. Accordingly, it now becomes desirable to improve the configuration of a thermal switch for utilization in more compact type discharge lamps such that the switch device occupies less space in the lamp yet still operates in a reliable manner.
It is an object of the present invention therefore to provide an improved thermal switch configuration particularly adapted for more compact discharge lamp utilization.
It is another object of the present invention to provide an improved thermal switch configuration of more compact design which also tolerates large excursions beyond the temperatures anticipated that the closure of the switch is to be subjected.
Still a different object of the present invention is to provide a more compact thermal switch device employing a thermally deformable metal element wherein substantially the entire length of the element is caused to bend for improved device actuation. These and still further objects of the present invention will become apparent upon considering the following detailed description of the present invention.